Catalytic cracking process using high equilibrium activity additive catalyst

ABSTRACT

A catalytic cracking process is disclosed in which octane improvement is attained by the addition to conventional cracking catalysts of small amounts of additive catalyst comprising a class of zeolites characterized by a silica to alumina mole ratio greater than about 12 and a Constraint Index of about 1 to 12 bound in a matrix chosen such that the matrix component forms a thermodynamically favored compound with selected cations. Sustained catalytic activity is achieved by pre-exchanging the catalyst to a high level of selected cation loading. By extending the active life of the additive catalyst, markedly lower makeup catalyst addition rates are required.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. Application Serial No. 130,491,filed Dec. 9, 1987 now U.S. Pat. No. 4,818,738.

FIELD OF THE INVENTION

This invention relates to an improved process to increase gasolineoctane number in catalytic cracking units by the addition of smallamounts of additive catalyst to cracking catalysts. The active life ofthe additive catalyst is extended by exchanging a portion of thecatalytically active sites with a cation selected such that the cationicform of the additive catalyst is thermodynamically favored undercatalyst synthesis conditions and the hydrogen form of the catalyst isfavored under catalytic cracking unit regenerator steaming conditions.

BACKGROUND OF THE INVENTION

Hydrocarbon conversion processes utilizing crystalline zeolites havebeen the subject of extensive investigation during recent years, as isobvious from both the patent and scientific literature. Crystallinezeolites have been found to be particularly effective for a wide varietyof hydrocarbon conversion processes including the catalytic cracking ofa gas oil to produce motor fuels and have been described and claimed inmany patents, including U.S. Pat. Nos. 3,140,249; 3,140,251; 3,140,252;3,140,253; and 3,271,418. The incorporation of a crystalline zeoliteinto a matrix for catalytic cracking is known and such disclosureappears in one or more of the above-identified U.S. patents.

In order to reduce automobile exhaust emissions to meet federal andstate pollution requirements, many automobile manufacturers haveequipped the exhaust systems of their vehicles with catalyticconverters. These converters contain catalysts which are poisoned bytetraethyl lead. Tetraethyl lead has been widely used to boost theoctane number of gasoline but may no longer be used. Refiners must nowturn to alternate means to improve gasoline octane number.

One method of increasing octane number is to raise the crackingtemperature. This method, however, is very limited, since many units arenow operating at maximum temperatures due to metallurgical limitations.Raising the cracking temperature also results in increased requirementsfor the gas plant (i.e. gas compressor and separator). Since most gasplants are now operating at maximum capacity, any increased load couldnot be tolerated by the present equipment.

As can well be appreciated from the foregoing, it would be extremelydesirable to have a process which will provide high octane unleadedgasoline without undue sacrifice of gasoline yield. It would be evenmore desirable if such results could be obtained in conjunction with amarked reduction in the use of expensive additive catalysts.

It is also known that improved results will be obtained with regard tothe catalytic cracking of gas oils if a crystalline zeolite having anintermediate pore size is included with a crystalline zeolite having alarge pore size, either with or without a matrix. A disclosure of thistype is found in U.S. Pat. No. 3,769,202.

Improved results in catalytic cracking with respect to both octanenumber and overall yield were achieved in U.S. Pat. No. 3,758,403. Insaid patent, the cracking catalyst was comprised of a large pore sizecrystalline zeolite in admixture with ZSM-5 type zeolite wherein theratio of ZSM-5 type zeolite to large pore size crystalline zeolite wasin the range of 1:10 to 3:1.

The use of ZSM-5 type zeolite in conjunction with a zeolite crackingcatalyst of the X or Y faujasite variety is described in U.S. Pat. Nos.3,894,931; 3,894,933; and 3,894,934. The two former patents disclose theuse of ZSM-5 type zeolite in amounts up to and about 5 to 10 weightpercent; the latter patent discloses the weight ratio of ZSM-5 typezeolite to large pore size crystalline zeolite in the range of 1:10 to3:1.

It is known that the addition of a very small amount of a medium-poresized zeolite additive catalyst to conventional cracking catalystsresults in a significant improvement in the octane number of theresultant gasoline while increasing the total yield comprised of C₅ ⁺gasoline and alkylate. U.S. Pat. No. 4,368,114 teaches this process,details its use in a fluidized catalytic cracking plant and isincorporated by reference as if set forth at length herein.

Before the advent of the present invention, it was accepted in theindustry that a certain amount of catalytic activity was lost due tosteaming during each catalyst regeneration. Additive catalyst additionrates were based on the rate of deactivation. The present unexpecteddiscovery, however, will dramatically reduce the amount of catalyticactivity lost during regeneration and will therefore reduce the additivecatalyst makeup requirements.

SUMMARY OF THE INVENTION

In accordance with the present invention, there has now been discoveredan improved process to upgrade the octane number in catalytic crackingunits while markedly reducing makeup additive catalyst requirements.

The additive catalyst of this invention comprises a class of zeoliteswhich are characterized by a silica to alumina mole ratio of at least12, having at least 10% of the exchangeable cation sites occupied byselected cations, preferably between 25 and 75% of the exchangeablecation sites occupied by selected cations, and a Constraint Index, ashereinafter described, of about 1 to 12. Cations useful in the presentinvention must first be sufficiently small to enter the pores of thezeolite catalyst to be exchanged. Second, the cationic form of thezeolite catalyst must be thermodynamically favored under the cationloading conditions for the particular zeolite while the hydrogen form ofthe zeolite catalyst is favored under catalytic cracking unitregenerator steaming conditions. Examples of such cations include thealkali metal cations, alkali earth cations, and transition metalcations.

The additive catalyst is bound in a matrix chosen such that the matrixcomponents forms a thermodynamically favored compound with the selectedcations under catalytic cracking unit regenerator steaming conditions.Aluminosilicate clays, for example, kaolin clays, are useful as matrixmaterial in the present invention. Clays which do not contain alumina,on the other hand, are not useful as matrix materials in the presentinvention. Some acid sites are generated during each regeneration cycleas the cations migrate and are irreversibly trapped by the matrixcomponent. The net effect of this timed release is increased catalystlife and, consequently, reduced catalyst make-up requirements.

The improved process of this invention affords the refiner greaterflexibility and economy in catalytic cracking operation, since only avery small quantity of additive catalyst can quickly boost the octanenumber of the product. The need for only very small quantities of thismakeup additive catalyst will also result in great savinga in catalystusage and will therefore result in more economic refinery operations.

DETAILED DESCRIPTION

Addition of a separate additive catalyst comprising one or more membersof a class of zeolites, as defined hereinafter, is extremely effectiveas an octane improver in very small amounts when used in conjunctionwith a conventional cracking catalyst. It has been found that theaddition of a member of this class of zeolites to the conventionalcracking catalyst in the unit under conventional cracking operations canincrease octane. Octane increase can be varied with the content of theadditive catalyst. The addition of the additive catalyst is useful toenhance octane at weight ratios of zeolite contained in the additivecatalyst to cracking catalyst of between about 1:1000 and 1:10,preferably between weight ratios of between about 1:200 and 1:20 zeolitecontained in the additive catalyst to cracking catalyst. If excessalkylation capacity is available, C₅ ⁺ gasoline plus alkylate yields arehigher when the additive catalyst is utilized as compared toconventional commercial cracking catalysts, without sacrificing theoctane increase.

The additive catalyst can be injected at any time during the catalyticcracking process. The additive catalyst can be introduced while thecracking unit is down, or while the cracking unit is on streamoperation. Once the additive catalyst is added to the cracking process,the refiner can return to conventional operation or any operation atlower octane number by eliminating or decreasing the use of additivecatalyst. Thus the increase in octane number over the number obtainableunder conventional cracking operations can be controlled by controllingthe amount of additive catalyst.

Catalytic cracking units which are amenable to the process of thisinvention operate within the temperature range of about 600° F. to 1300°F. and under reduced atmospheric or superatmospheric pressure. Thecatalytic cracking process may be operated batchwise or continuously.The catalytic cracking process can be either fixed bed, moving bed orfluidized bed and the hydrocarbon charge stock flow may be eitherconcurrent or countercurrent to the conventional catalyst flow. Theprocess of this invention is particularly applicable to the fluidcatalytic cracking (FCC) process.

The amount of additive catalyst required to increase gasoline octanenumber is generally based on the total quantity of conventional crackingcatalyst in the unit, i.e. on the circulating inventory of conventionalcracking catalyst. For example, if the additive catalyst is firstintroduced via the addition of fresh makeup catalyst, the amount ofzeolite constituent in the additive catalyst required would be quitehigh if compared against the amount of fresh makeup catalyst added.However, after a period of time of fresh makeup catalyst addition, andonce the amount of zeolite in the additive catalyst is maintained at theprescribed limits as compared to the circulating inventory ofconventional cracking catalyst, the amount of the zeolite in the freshmakeup catalyst addition will be much lower than initially. In actualoperation, because the catalytic activity of the circulating inventoryof catalyst tends to decrease with age, fresh makeup catalyst is addedto maintain optimal catalyst activity.

Hydrocarbon charge stocks undergoing cracking in accordance with thisinvention comprise hydrocarbons generally and, in particular, petroleumfractions having an initial boiling point range of at least 400° F., a50% point range of at least 500° F. and an end point range of at least600° F. Such hydrocarbon fractions include gas oils, residual oils,cycle stocks, whole top crudes and heavy hydrocarbon fractions derivedby the destructive hydrogenation of coal, tar, pitches, asphalts and thelike. As will be recognized, the distillation of higher boilingpetroleum fractions above about 750° F. must be carried out under vacuumin order to avoid thermal cracking. The boiling temperatures utilizedherein are expressed in terms in convenience of the boiling pointcorrected to atmospheric pressure.

The members of the class of zeolites of the additive catalyst constitutean unusual class of natural and synthetic minerals. They arecharacterized by having a rigid crystalline framework structure composedof an assembly of silicon and aluminum atoms, each surrounded by atetrahedron of shared oxygen atoms, and a precisely defined porestructure. Exchangeable cations are present in the pores.

The additive catalysts referred to herein utilize members of a class ofzeolites exhibiting some unusual properties. These zeolites induceprofound transformations of aliphatic hydrocarbons to aromatichydrocarbons in commercially desirable yields and are generally highlyeffective in alkylation, isomerization, disproportionation and otherreactions involving aromatic hydrocarbons. Although they have unusuallylow alumina contents, i.e. high silica to alumina mole ratios, they arevery active even with silica to alumina mole ratios exceeding 30. Thisactivity is surprising, since catalytic activity of zeolites isgenerally attributed to framework aluminum atoms and cations associatedwith these aluminum atoms. These zeolites retain their crystallinity forlong periods in spite of the presence of steam even at high temperatureswhich induce irreversible collapse of the crystal framework of otherzeolites, e.g. of the X and A type. Furthermore, carbonaceous deposits,when formed, may be removed by burning at higher than usual temperaturesto restore activity. In many environments, the zeolites of this classexhibit very low coke forming capability, conducive to very long timeson stream between burning regenerations.

An important characteristic of the crystal structure of this class ofzeolites is that it provides constrained access to, and egress from, theintracrystalline free space by virtue of having a pore dimension greaterthan about 5 Angstroms and pore windows of about a size such as would beprovided by 10-membered rings of oxygen atoms. It is to be understood,of course, that these rings are those formed by the regular dispositionof the tetrahedra making up the anionic framework of the crystallinezeolite, the oxygen atoms themselves being bonded to the silicon oraluminum atoms at the centers of the tetrahedra. Briefly, the preferredzeolites useful in the additive catalysts of this invention possess, incombination: a Constraint Index, as hereinafter defined, of about 1 to12, a silica to alumina mole ratio of at least about 12, and a structureproviding constrained access to the intracrystalline free space.

The silica to alumina mole ratio referred to may be determined byconventional analysis. This ratio is meant to represent, as closely aspossible, the ratio in the rigid anionic framework of the zeolitecrystal and to exclude aluminum in the binder or in cationic or otherform within the channels.

Although additive catalysts comprising zeolites with a silica to aluminamole ratio of at least about 12 are useful, it is preferred to usezeolites having higher ratios of at least about 30. In some zeolites,the upper limit of silica to alumina mole ratio is unbounded, withvalues of 30,000 and greater, extending at least theoretically up toinfinity. Therefore, the silica to alumina mole ratio of the zeolitecomponent of the additive catalyst for use herein may be from about 12to infinity, preferably from about 30 to infinity. Such zeolites, afteractivation, acquire an intracrystalline sorption capacity for normalhexane which is greater than that for water, i.e. they exhibit"hydrophobic" properties. It is believed that this hydrophobic characteris advantageous in the present invention.

The zeolites comprising the additive catalysts in this invention freelysorb normal hexane and have a pore dimension greater than about 5Angstroms. In addition, their structure must provide constrained accessto some larger molecules. It is sometimes possible to judge from a knowncrystal structure whether such constrained access exists. For example,if the only pore windows in a crystal are formed by 8-membered rings ofoxygen atoms, then access by molecules of larger cross-section thannormal hexane is substantially excluded and the zeolite is not of thedesired type. Additive catalysts with zeolites with windows of 10-memberrings are preferred, although excessive puckering or pore blockage mayrender these zeolites substantially ineffective.

Additive catalysts comprising zeolites with windows of 12-membered ringsdo not generally appear to offer sufficient constraint to produce theadvantageous conversions desired in the instant invention, althoughstructures can be conceived, due to pore blockage or other cause, thatmay be operative.

Rather than attempt to judge from crystal structure whether or not azeolite possesses the necessary constraint access, a simpledetermination of the "Constraint Index" may be made. U.S. Pat. No.4,016,218 details the significance of and the procedure for determiningConstraint Index and is incorporated by reference as if set forth atlength herein. U.S. Pat. No. 4,696,732 details typical values of theConstraint Index for various zeolites and is incorporated by referenceas if set forth at length herein.

The additive catalysts of this invention may be prepared in variousways. The additive catalysts may be separately prepared in the form ofparticles such as pellets or extrudates, for example, and simply mixedin the required proportions. The particle size of the individualcomponent particles may be quite small, for example from about 20 toabout 150 microns, when intended for use in fluid bed operation, or theymay be as large as up to about 1/2 inch for fixed bed operation. Or thecomponents may be mixed as powders and formed into pellets or extrudate,each pellet containing both components in substantially the requiredproportions.

As is the case of many catalysts, it is desirable to incorporate thezeolite component of the additive catalyst in a matrix. Such matrix isuseful as a binder and imparts greater resistance to the catalyst forthe severe temperature, pressure and velocity conditions encountered inmany cracking processes.

Matrix materials include both synthetic and natural substances. Suchsubstances include clays, silica and/or metal oxides. The latter may beeither naturally occurring or in the form of gelatinous precipitates,sols or gels including mixtures of silica and metal oxides. Frequently,zeolite materials have been incorporated into naturally occurring clays,e.g. bentonite and kaolin.

In addition to the foregoing materials, the zeolite for use herein canbe composited with a porous matrix material such as silica-alumina,silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia,silica-titania, as well as ternary compositions such assilica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesiaand silica-magnesia-zirconia. The matrix can be in the form of a cogel.A mixture of clay in combination with silica or any of the abovespecified cogels to form a matrix may also be used.

Cracking catalysts contain active components which may be zeolitic ornon-zeolitic. The non-zeolitic active components are generally amorphoussilica-alumina and crystalline silica-alumina. However, the majorconventional cracking catalysts presently in use generally comprise acrystalline zeolite (active component) in a suitable matrix.Representative crystalline zeolite active component constituents ofcracking include zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S.Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeoliteZK-4 (U.S. Pat. No. 3,314,752), synthetic mordenite and dealuminizedsynthetic mordenite, merely to name a few, as well as naturallyoccurring zeolites, including faujasite, mordenite, and the like.Preferred crystalline zeolites include the synthetic faujasite zeolitesX and Y, with particular preference being accorded zeolite Y.

In the present invention, cations are exchanged with some of thehydrogen sites inside the pores of the zeolite catalyst. The particularcations to be exchanged must be selected to be compatible with thezeolite catalyst. First, the cations must be small enough to enter thepores of the zeolite to exchange with the available sites. Second, thecationic form of the zeolite must be preferred over the hydrogen formunder cation loading conditions for the particular zeolite while thehydrogen form of the zeolite catalyst is favored under catalyticcracking unit regenerator steaming conditions. Examples of cations whichare useful for exchange with various zeolites include the alkali metalcations, alkali earth cations, and transition metal cations.

The crystalline zeolite additive is prepared by exchanging from 10 to100% of the catalytically active acid sites with cations selected asdescribed above. The cations protect the bulk of the acid sites fromdeactivation during steaming in the catalytic cracking unit regenerator.At the same time a portion of the cations migrate from the crystallinezeolite to the matrix thereby releasing some acid sites for reaction inthe catalytic cracking unit riser section. This cation movement ispossible because the cation is chosen such that the hydrogen form ofcrystalline zeolite is favored under regenerator steaming conditions.The cations are irreversibly trapped in the catalyst matrix by acomponent (e.g. clay, silica, or metal oxides) chosen such that thematrix component forms a thermodynamically favored compound with thecation under catalytic cracking unit regenerator steaming conditions.This process repeats as the cations gradually move from the crystallinezeolite to the matrix with each pass through the regenerator. The neteffect is a sustained release of crystalline zeolite acid activity.Although the cation exchange results in somewhat lower initial catalystactivity, the additive maintains its activity for a longer time. Thisreduces the make-up requirement.

It is preferred to have the crystalline zeolite of the cracking catalystin a suitable matrix, since this catalyst form is generallycharacterized by a high resistance to attrition, high activity andexceptional steam stability. Such catalysts are readily prepared bydispersing the crystalline zeolite in a suitable siliceous sol andgelling the sol by various means. The inorganic oxide which serves asthe matrix in which the above crystalline zeolite is distributedincludes silica gel or a cogel of silica and a suitable metal oxide.Representative cogels include silica-alumina, silica-magnesia,silica-zirconia, silica-thoria, silica-beryllia, silica-titania, as wellas ternary combinations such as silica-alumina-magnesia,silica-alumina-zirconia and silica-magnesia-zirconia. Preferred cogelsinclude silica-alumina, silica-zirconia or silica-alumina-zirconia. Theabove gels and cogels will generally comprise a major proportion ofsilica and a minor proportion of the other aforementioned oxide oroxides. Thus, the silica content of the siliceous gel or cogel matrixwill generally fall within the range of 55 to 100 weight percent,preferably 60 to 95 weight percent, and the other metal oxide or oxidescontent will generally be within the range of 0 to 45 weight percent andpreferably 5 to 40 weight percent. In addition to the above, the matrixmay also comprise natural or synthetic clays, such as kaolin type clays,montmorillonite, bentonite or halloysite. These clays may be used eitheralone or in combination with silica or any of the above specified cogelsin matrix formulation.

Where a matrix is used, content of catalytically active component, e.g.crystalline zeolite, i.e. the amount of the zeolite Y component, in theconventional cracking catalyst, is generally at least about 5 weightpercent, and more particularly between about 5 and about 50 weightpercent. Ion exchange of the zeolite to replace its initial cationcontent can be accomplished either prior to or subsequent toincorporation of the zeolite into the matrix.

Where no matrix as such is used, such as where a non-zeolitic crackingcatalyst, e.g. silica-alumina, is used, content of catalytically activecomponent will, of course, approach 100 weight percent. Also, sincesilica-alumina may serve as a matrix material for catalytically activezeolite component, 100 weight percent catalytically active catalyst mayexist.

The above compositions may be readily processed so as to provide fluidcracking catalysts by spray drying the composite to form microspheroidalparticles of suitable size. Alternatively, the composition may beadjusted to suitable concentration and temperature to form bead typecatalyst particles suitable for use in moving bed type cracking systems.The catalyst may also be used in various other forms such as thoseobtained by tabletting, balling or extruding.

The following examples will serve to illustrate the invention.

EXAMPLE 1

This example will illustrate the catalytic cracking of a gas oil with acracking catalyst. There is no additive catalyst used in this example. Asample of equilibrium REY catalyst was tested in a fixed fluidized-bedreactor with a sour heavy gas oil, (properties given in Table 1). Theperformance of this catalyst was used as a base case for comparison withZSM-5 containing catalyst samples. The results are shown in Table 2.

                  TABLE 1                                                         ______________________________________                                                           Sour Heavy                                                 Chargestock        Gas Oil                                                    ______________________________________                                        Gravity, °API                                                                             24.3                                                       Aniline Pt., °F.                                                                          171                                                        Sulfur, wt. %      1.87                                                       Nitrogen, wt. %    0.10                                                       Basic Nitrogen, ppm                                                                              327                                                        Conradson Carbon, wt. %                                                                          0.28                                                       Viscosity, KV at 210° F.                                                                  3.6                                                        Bromine No.        4.2                                                        R.I. at 70° F.                                                                            1.5080                                                     Hydrogen, wt. %    12.3                                                       Molecular Weight   358                                                        Pour Point, °F.                                                                           85                                                         Paraffins, wt. %   23.5                                                       Naphthenes, wt. %  32.0                                                       Aromatics, wt. %   44.5                                                       C.sub.A, wt. %     18.9                                                       ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Catalyst           Equil. REY                                                 ______________________________________                                        Conversion, vol. % 65                                                         C.sub.5.sup.+ Gasoline Yld, vol. %                                                               50.4                                                       RON + 0            89.5                                                       i-C.sub. 4 + C.sub.3.sup.=  + C.sub.4.sup.= vol. %                                               20.2                                                       C.sub.3.sup.- Gas, wt. %                                                                         7.4                                                        ______________________________________                                    

EXAMPLE 2

This example will serve to illustrate the beneficial effects of theadditive catalyst in conventional cracking processes. A 25 wt. % ZSM-5additive sample in a kaoline clay containing matrix (<0.1 wt. % Na) wassteamed at 1450° F. for 10 hours under atmospheric pressure with a 45/55steam air mixture. An 8 wt. % blend of this catalyst and the equilibriumREY catalyst described in Example 1 was tested in a fixed fluidized-bedbench unit with the sour heavy gas oil shown in Table 1. The 8 wt. %blend of this composite catalyst represents a zeolite additive catalystto cracking catalyst weight ratio of 1:50. The results are shown inTable 3 with changes from the base case of Example 1, Table 2 designatedby "Δ".

                  TABLE 3                                                         ______________________________________                                        Catalyst            Equil. REY + ZSM-5                                        ______________________________________                                        Conversion, vol. %  65                                                        C.sub.5.sup.+ Gasoline Yld, vol. %                                                                47.5                                                      RON + 0             90.8                                                      i-C.sub. 4 + C.sub.3.sup.=  + C.sub.4.sup.=, vol. %                                               23.9                                                      C.sub.3 -Gas, wt. % 8.9                                                       Coke, wt. %         4.5                                                       Gasoline + Alkylate, vol. %                                                                       76.1                                                      Δ Gasoline, vol. %                                                                          -2.9                                                      Δ RON + 0     +1.3                                                      Δ (i-C.sub. 4 + C.sub.3.sup.=  + C.sub.4.sup.=), vol.                                       +3.7                                                      ______________________________________                                    

EXAMPLE 3

This example will serve to illustrate the beneficial effects of theadditive catalyst of this invention in conventional catalytic crackingprocesses. The 25 wt. % additive described in Example 2 was impregnatedwith 0.54 wt. % sodium added to the catalyst as a sodium nitratesolution in water. The catalyst was then steamed as in Example 2,blended with the equilibrium REY base case catalyst and evaluated withthe sour heavy gas oil as described above. The results are shown inTable 4 with changes from the base case of Example 1, Table 2 designatedby "Δ".

                  TABLE 4                                                         ______________________________________                                                            Equil. REY +                                              Catalyst            Na ZSM-5                                                  ______________________________________                                        Conversion, vol. %  65                                                        C.sub.5.sup.+  Gasoline Yld, vol. %                                                               46.1                                                      RON + 0             91.4                                                      i-C.sub. 4 + C.sub.3.sup.= + C.sub.4.sup.=, vol. %                                                25.2                                                      C.sub.3.sup.-  Gas, wt. %                                                                         8.9                                                       Coke, wt. %         4.8                                                       Gasoline + Alkylate, vol. %                                                                       76.4                                                      Δ Gasoline, vol. %                                                                          -4.3                                                      Δ RON + 0     +1.9                                                      Δ (i-C.sub. 4 + C.sub.3.sup.=  + C.sub.4.sup.=), vol.                                       +5.0                                                      ______________________________________                                    

Relative to the base case, the catalyst in Example 3 gave a higheroctane boost, higher i-C₄ +C₃ ⁼ +C₄ ⁼ yield and a lower gasoline yieldthan the catalyst in Example 2. This indicates that Na exchange prior tosteaming stabilized the ZSM-5 additive. As shown in Example 3, thesodium cations provide protection against catalyst deactivationresulting from steaming during regeneration.

Changes and modifications in the specifically described embodiments canbe carried out without departing from the scope of the invention whichis intended to be limited only by the scope of the appended claims.

What is claimed is:
 1. A process for the catalytic cracking of ahydrocarbon feedstock under cracking conversion conditions in thepresence of a mixture of catalysts comprising:(i) a cracking catalyst,and (ii) an additive catalyst comprising a crystalline zeolite having aConstraint Index of from about 1 to about 12 and having at least 10% ofthe exchangeable cation sites occupied by cations, said cations selectedsuch that the cationic form of said zeolite is thermodynamically favoredat cation loading conditions and the hydrogen form of said zeolite isthermodynamically favored under catalytic cracking unit regeneratorsteaming conditions, said zeolite bound in a matrix chosen such that thematrix component forms a thermodynamically favored compound with saidcations under catalytic cracking unit regenerator steaming conditionswherein the maintenance of the activity of the additive zeolite isenhanced.
 2. A process for the catalytic cracking of a hydrocarbonfeedstock under cracking conversion conditions in the presence of amixture of catalysts comprising:(i) a large pore size cracking catalyst,and (ii) an additive catalyst comprising a crystalline zeolite having aConstraint Index of from about 1 to about 12 and having at least 10% ofthe exchangeable cation sites occupied by alkali metal cations, bound ina matrix chosen such that the matrix component forms a thermodynamicallyfavored compound with said alkali metal cations under catalytic crackingunit regenerator steaming conditions wherein the maintenance of theactivity of the additive zeolite is enhanced.
 3. The process of claim 2wherein said additive catalyst further comprises a crystalline zeolitehaving between 25 and 75% of the exchangeable cation sites occupied byalkali metal cations.
 4. The process of claim 2 wherein said matrixcomprises a clay.
 5. The process of claim 2 wherein said alkali metalcation is sodium.
 6. The process of claim 2 wherein said additivecatalyst comprises ZSM-5.
 7. The process of claim 2 wherein the weightratio of said zeolite contained in said additive catalyst to saidcracking catalyst is between about 1:1000 and 1:10.
 8. The process ofclaim 2 wherein the weight ratio of said zeolite contained in saidadditive catalyst to said cracking catalyst is between bout 1:200 and1:20.
 9. A process for the catalytic cracking of a hydrocarbon feedstockunder cracking conditions in the presence of a mixture of catalystscomprising:(i) a large pore size cracking catalyst, and (ii) an additivecatalyst comprising a crystalline zeolite having a Constraint Index offrom about 1 to about 12 and having at least 10% of the exchangeablecation sites occupied by cations, said cations selected from the groupconsisting of alkali metal cations, alkali earth cations, transitionmetal cations and mixtures thereof, said zeolite bound in a matrixselected from the group consisting of silica gels, cogels of silica andmetal oxides, natural clays, synthetic clays and mixtures thereof,wherein the maintenance of the activity the additive zeolite isenhanced.
 10. The process of claim 8 wherein said additive catalystfurther comprises a crystalline zeolite having between 25 and 75% of theexchangeable cation sites occupied by alkali metal cations.
 11. Theprocess of claim 10 wherein said matrix comprises a clay.
 12. Theprocess of claim 10 wherein said alkali metal cation is sodium.
 13. Theprocess of claim 10 wherein said additive catalyst comprises ZSM-5. 14.The process of claim 10 wherein the weight ratio of said zeolitecontained in said additive catalyst to said cracking catalyst is betweenabout 1:1000 and 1:10.
 15. The process of claim 10 wherein the weightratio of said zeolite contained in said additive catalyst to saidcracking catalyst is between about 1:200 and 1:20.